The conglutinating complement absorption microtest and passive hemagglutination test were applied to the measurement of antibodies to feline leukemia virus (FeLV) in cat sera. We have found that cat antibodies to FELV fixed guinea pig complement poorly in the usual complement fixation test, but absorbed horse complement well in the conglutinating complement absorption system. Using sheep erythrocytes sensitized with chromatographically purified fractions of FeLV we were able to compare immune responses to FeLV of heterologous and homologous species of animals. Experiments were also set up to examine the serologic status of cats with naturally occurring leukemias, cats which had been hyperimmunized with FeLV antigen, and a number of normal cats. Various tests were performed including electron microscopic examinations for the presence of C-type virus, indirect fluorescent antibody test for FeLV or FeLV-associated cell membrane antigen, focus reduction neutralization, complement fixation, conglutinating complement absorption, immunodiffusion, and passive hemagglutination techniques.